一篇带给你EventBus原理解析

开发 前端
EventBus基于观察者模式,原理是大体也是将观察者与要观察的消息注册到一个Map,当有消息发布时,从Map中找到观察者,并调用观察者对应的方法,下面我们基于源码进行解析来看看是不是这样的原理。

EventBus基于观察者模式,原理是大体也是将观察者与要观察的消息注册到一个map,当有消息发布时,从map中找到观察者,并调用观察者对应的方法,下面我们基于源码进行解析来看看是不是这样的原理

不废话,没有卖课,直接来

1、EventBus订阅

(1)订阅整体流程

EventBus的定义始于register方法:

public void register(Object subscriber) {
//获取订阅者/观察者的class对象
Class<?> subscriberClass = subscriber.getClass();
//从订阅者class对象中找出所有订阅消息的方法列表
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
//变量订阅消息的方法列表
//定义消息
subscribe(subscriber, subscriberMethod);
}
}
}

register方法主要是从订阅者class对象中找出所有订阅消息的方法列表,然后对每个订阅方法调用subscribe进行订阅:

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//方法订阅的消息类型,即订阅方法的参数类型class
Class<?> eventType = subscriberMethod.eventType;
//创建订阅者/观察者
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//从事件/订阅者map中,找出订阅事件的订阅者列表
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
//事件/订阅者map中没有事件相关的订阅者,则:
//新建订阅事件的订阅者列表
subscriptions = new CopyOnWriteArrayList<>();
//并将事件的class作为key,事件订阅者列表作为value存入事件/订阅者map
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//
if (subscriptions.contains(newSubscription)) {
//已经订阅过,则抛出异常
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
int size = subscriptions.size();
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
//按优先级将订阅者加入对应的位置
subscriptions.add(i, newSubscription);
break;
}
}
//订阅者/事件map中找到订阅事件列表
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
//如果订阅者/事件map没有订阅事件列表,则:
//新建订阅事件列表,并存入订阅者/事件map中,key是订阅者,value是要订阅的事件列表
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
if (subscriberMethod.sticky) {
//如果订阅事件是粘性事件,则:
if (eventInheritance) {
//事件允许发布到其父类或者父接口,则:
// Existing sticky events of all subclasses of eventType have to be considered.
// Note: Iterating over all events may be inefficient with lots of sticky events,
// thus data structure should be changed to allow a more efficient lookup
// (e.g. an additional map storing sub classes of super classes: Class -> List<Class>).
Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
for (Map.Entry<Class<?>, Object> entry : entries) {
//遍历粘性事件map
Class<?> candidateEventType = entry.getKey();
if (eventType.isAssignableFrom(candidateEventType)) {
//事件是粘性事件的父类、父接口,则:
Object stickyEvent = entry.getValue();
//发布粘性事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
} else {
//发布粘性事件
Object stickyEvent = stickyEvents.get(eventType);
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}


  • subscribe订阅事件主要是将订阅者与要订阅的事件存入事件/订阅者map中,等待发布消息时从map中找到订阅者并调用对应的订阅方法消耗处理事件。
  • subscribe订阅时有两个map,一个是subscriptionsByEventType(事件/订阅者map,key是事件,value是订阅者列表),一个是typesBySubscriber(订阅者/事件列表map,key是订阅者,value是要订阅的事件列表),subscriptionsByEventType用于发布消息时从map中快速找到订阅者列表并调用相应的订阅方法,typesBySubscriber用于取消订阅时,从map中快速找到事件列表,根据此事件列表快速从subscriptionsByEventType取消订阅。
  • 订阅消息时确实也是将订阅者与定要订阅的事件存入事件/订阅者map中。

register方法中的subscriberMethodFinder.findSubscriberMethods(subscriberClass),它从订阅者class对象中找到要所有要订阅的事件,我们看看它是怎么实现的:

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//从缓存中获取订阅者的所有事件方法列表
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//缓存汇总没有,则:
if (ignoreGeneratedIndex) {
//如果忽略从编译器期自动生成订阅者/事件关系文件中查找,则:
//通过反射订阅者class找到订阅事件方法列表
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//如果从编译器期自动生成订阅者/事件关系文件中查找,则:
//从订阅者/事件关系文件中查找订阅事件方法列表
subscriberMethods = findUsingInfo(subscriberClass);
}
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
//将订阅者的订阅事件列表进行缓存
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
  • findSubscriberMethods有两个策略。
  • 一是通过反射订阅者class对象找到所有的订阅事件方法列表。
  • 一是从编译期自动生成的订阅者/事件关系文件中查找,默认是从订阅者/事件关系文件中查找。

(2)反射查找订阅事件方法列表

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
//从查找状态对象复用池中获取/创建一个新的查找状态对象
FindState findState = prepareFindState();
//初始化查找状态对象,设置订阅者class
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//通过反射查找查找状态对象当前class,最开始从订阅者class开始查找,接着递归其父类或者父接口查找
findUsingReflectionInSingleClass(findState);
//继续从其父类/父接口中查找
findState.moveToSuperclass();
}
//返回订阅事件方法列表,并将查找状态对象缓存到复用池中
return getMethodsAndRelease(findState);
}
  • findUsingReflection会从当前订阅者的class开始查找所有的订阅事件方法列表;
  • 接着递归其父类或者父接口继续查找,真正查找一个class的订阅事件方法列表是findUsingReflectionInSingleClass方法:
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
//反射获取当前class定义的方法列表
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
// Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
//出现异常,则获取所有方法(包含所有父类/父接口)
methods = findState.clazz.getMethods();
//并标记不需要从父类/父接口汇总查找
findState.skipSuperClasses = true;
}
for (Method method : methods) {
//遍历方法列表
//获取方法访问性
int modifiers = method.getModifiers();
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
//方法是public、静态、抽象,则:
//反射获取方法的参数类型列表
Class<?>[] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 1) {
//方法参数只有一个参数,则:
//反射获取方法的Subscribe注解对象
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
//方法被Subscribe注解修饰,则:
//获取参数类型class
Class<?> eventType = parameterTypes[0];
if (findState.checkAdd(method, eventType)) {
//获取事件方法的线程模型
ThreadMode threadMode = subscribeAnnotation.threadMode();
//收集注解信息:线程模型、优先级、粘性,创建事件方法,并加入订阅者的订阅事件方法列表中
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
//如果方法严格校验并且被Subscribe注解修饰,则抛出方法参数多于1个的异常
String methodName = method.getDeclaringClass().getName() + "." + method.getName();
throw new EventBusException("@Subscribe method " + methodName +
"must have exactly 1 parameter but has " + parameterTypes.length);
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
//如果方法严格校验并且被Subscribe注解修饰,则抛出方法非public、非静态、非抽象的异常
String methodName = method.getDeclaringClass().getName() + "." + method.getName();
throw new EventBusException(methodName +
" is a illegal @Subscribe method: must be public, non-static, and non-abstract");
}
}
}

findUsingReflectionInSingleClass通过反射从class中获取方法列表,并按Subscribe注解规则(规则是:方法被Subscribe修饰、是public/static/abstract、只有一个参数)进行过滤,找到匹配的事件方法,并加入订阅事件方法列表

(3)编译期注解处理器,从编译期生成的订阅者/事件关系文件中查找订阅方法列表

  • 从编译期生成的订阅者/事件关系文件中查找订阅方法列表,涉及编译期注解处理器解析注解生成订阅者/事件方法列表关系文件,然后运行时从这些文件中根据订阅者class直接找到事件方法列表。
  • EventBus的编译期注解处理器是独立的模块,依赖使用它可以在编译期自动解析生成订阅者/事件方法列表关系文件,避免运行时通过反射查找事件方法列表的性能开销.

EventBusAnnotationProcessor 是EventBus的编译期注解处理器实现类,其核心源码如下:

//订阅者/事件方法列表关系列表
private final ListMap<TypeElement, ExecutableElement> methodsByClass = new ListMap<>();
//被忽略的类
private final Set<TypeElement> classesToSkip = new HashSet<>();
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment env) {
Messager messager = processingEnv.getMessager();
try {
//读取模块定义的订阅者/事件索引类名(含包名+类名)
String index = processingEnv.getOptions().get(OPTION_EVENT_BUS_INDEX);
if (index == null) {
messager.printMessage(Diagnostic.Kind.ERROR, "No option " + OPTION_EVENT_BUS_INDEX +
" passed to annotation processor");
return false;
}
verbose = Boolean.parseBoolean(processingEnv.getOptions().get(OPTION_VERBOSE));
int lastPeriod = index.lastIndexOf('.');
//订阅者/事件索引类包名
String indexPackage = lastPeriod != -1 ? index.substring(0, lastPeriod) : null;
//......
//通过注解收集订阅者/事件方法列表关系列表
collectSubscribers(annotations, env, messager);
//根据订阅语法规则,生成忽略被忽略的观察者列表
checkForSubscribersToSkip(messager, indexPackage);
if (!methodsByClass.isEmpty()) {
//生成订阅者/事件索引类文件
createInfoIndexFile(index);
} else {
messager.printMessage(Diagnostic.Kind.WARNING, "No @Subscribe annotations found");
}
writerRoundDone = true;
} catch (RuntimeException e) {
// IntelliJ does not handle exceptions nicely, so log and print a message
e.printStackTrace();
messager.printMessage(Diagnostic.Kind.ERROR, "Unexpected error in EventBusAnnotationProcessor: " + e);
}
return true;
}
//通过注解收集订阅者/事件方法列表关系列表
private void collectSubscribers(Set<? extends TypeElement> annotations, RoundEnvironment env, Messager messager) {
for (TypeElement annotation : annotations) {
//遍历注解类型列表
//从注解类型元信息中获取匹配的注解元信息列表
Set<? extends Element> elements = env.getElementsAnnotatedWith(annotation);
for (Element element : elements) {
//遍历注解元信息列表
if (element instanceof ExecutableElement) {
//如果是方法元信息,则:
ExecutableElement method = (ExecutableElement) element;
if (checkHasNoErrors(method, messager)) {
//校验方法元信息,如果方法是public且只有一个参数,则:
//获取方法所属的类类型元信息,此类类型就是对应的订阅者,方法是消耗处理事件
TypeElement classElement = (TypeElement) method.getEnclosingElement();
//将订阅者/事件方法加入订阅者/事件方法列表map中,key是订阅者,value是事件方法列表
methodsByClass.putElement(classElement, method);
}
} else {
messager.printMessage(Diagnostic.Kind.ERROR, "@Subscribe is only valid for methods", element);
}
}
}
}
private void checkForSubscribersToSkip(Messager messager, String myPackage) {
for (TypeElement skipCandidate : methodsByClass.keySet()) {
//遍历订阅者/事件方法列表map
//订阅者class类类型元信息
TypeElement subscriberClass = skipCandidate;
while (subscriberClass != null) {
//循环:订阅者class不为空,则:
if (!isVisible(myPackage, subscriberClass)) {
//如果订阅者class不是public或者与要生成的索引类不是同包,则忽略此订阅则,将其加入忽略列表
boolean added = classesToSkip.add(skipCandidate);
//.......
break;
}
//取出订阅者要订阅的事件方法列表
List<ExecutableElement> methods = methodsByClass.get(subscriberClass);
if (methods != null) {
for (ExecutableElement method : methods) {
//遍历订阅的事件方法列表
String skipReason = null;
VariableElement param = method.getParameters().get(0);
TypeMirror typeMirror = getParamTypeMirror(param, messager);
if (!(typeMirror instanceof DeclaredType) ||
!(((DeclaredType) typeMirror).asElement() instanceof TypeElement)) {
//参数类型无法处理(不是基类性、类/接口类型、数组类型、类型变量、空类型),则标间忽略原因,一般不会出现此情况
skipReason = "event type cannot be processed";
}
if (skipReason == null) {
TypeElement eventTypeElement = (TypeElement) ((DeclaredType) typeMirror).asElement();
if (!isVisible(myPackage, eventTypeElement)) {
//如果参数类型对应索引类不可访问:如不是public或者不在同包名等,则标间忽略原因
skipReason = "event type is not public";
}
}
if (skipReason != null) {
//如果被标记被忽略,则:
//加入忽略列表
boolean added = classesToSkip.add(skipCandidate);
//.......
break;
}
}
}
//获取订阅类的父类/父接口,继续递归过滤
subscriberClass = getSuperclass(subscriberClass);
}
}
}
//生成订阅者/事件方法列表索引类文件
private void createInfoIndexFile(String index) {
BufferedWriter writer = null;
try {
//生成订阅者/事件方法列表索引类文件,文件为索引类全限定名
JavaFileObject sourceFile = processingEnv.getFiler().createSourceFile(index);
int period = index.lastIndexOf('.');
//索引类包名
String myPackage = period > 0 ? index.substring(0, period) : null;
//索引类名
String clazz = index.substring(period + 1);
//创建文件读写器
writer = new BufferedWriter(sourceFile.openWriter());
if (myPackage != null) {
//文件写入类包名
writer.write("package " + myPackage + ";\n\n");
}
//文件写入引用的类
writer.write("import org.greenrobot.eventbus.meta.SimpleSubscriberInfo;\n");
writer.write("import org.greenrobot.eventbus.meta.SubscriberMethodInfo;\n");
writer.write("import org.greenrobot.eventbus.meta.SubscriberInfo;\n");
writer.write("import org.greenrobot.eventbus.meta.SubscriberInfoIndex;\n\n");
writer.write("import org.greenrobot.eventbus.ThreadMode;\n\n");
writer.write("import java.util.HashMap;\n");
writer.write("import java.util.Map;\n\n");
writer.write("/** This class is generated by EventBus, do not edit. */\n");
//文件写入定义类并实现SubscriberInfoIndex 接口
writer.write("public class " + clazz + " implements SubscriberInfoIndex {\n");
//文件写入声明订阅者/事件方法列表map,key是订阅者class,value是SubscriberInfo订阅信息对象
writer.write(" private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;\n\n");
//文件写入静态块
writer.write(" static {\n");
//文件写入创建订阅者/事件方法列表map
writer.write(" SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();\n\n");
//调用writeIndexLines完成将订阅者/事件方法列表写入map中
writeIndexLines(writer, myPackage);
writer.write(" }\n\n");
//文件写入putIndex方法,方法用于将订阅者要订阅的信息存入map中
writer.write(" private static void putIndex(SubscriberInfo info) {\n");
writer.write(" SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);\n");
writer.write(" }\n\n");
//文件写入实现SubscriberInfoIndex 接口的getSubscriberInfo方法,方法是根据订阅者class从map中取订阅者信息并返回
writer.write(" @Override\n");
writer.write(" public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {\n");
writer.write(" SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);\n");
writer.write(" if (info != null) {\n");
writer.write(" return info;\n");
writer.write(" } else {\n");
writer.write(" return null;\n");
writer.write(" }\n");
writer.write(" }\n");
writer.write("}\n");
} catch (IOException e) {
throw new RuntimeException("Could not write source for " + index, e);
} finally {
if (writer != null) {
try {
writer.close();
} catch (IOException e) {
//Silent
}
}
}
}
//完成将订阅者/事件方法列表写入map中
private void writeIndexLines(BufferedWriter writer, String myPackage) throws IOException {
for (TypeElement subscriberTypeElement : methodsByClass.keySet()) {
//遍历收集到的订阅者/事件方法列表
if (classesToSkip.contains(subscriberTypeElement)) {
//订阅者被忽略
continue;
}
//获取订阅者class
String subscriberClass = getClassString(subscriberTypeElement, myPackage);
if (isVisible(myPackage, subscriberTypeElement)) {
//调用索引类的putIndex方法,创建订阅信息类SimpleSubscriberInfo,作为参数,其作用是写入如下语句:
putIndex(new SimpleSubscriberInfo(subscriberClass, true, SubscriberMethodInfo[]))
writeLine(writer, 2,
"putIndex(new SimpleSubscriberInfo(" + subscriberClass + ".class,",
"true,", "new SubscriberMethodInfo[] {");
List<ExecutableElement> methods = methodsByClass.get(subscriberTypeElement);
writeCreateSubscriberMethods(writer, methods, "new SubscriberMethodInfo", myPackage);
writer.write(" }));\n\n");
} else {
writer.write(" // Subscriber not visible to index: " + subscriberClass + "\n");
}
}
}
//创建SubscriberMethodInfo对象并写入索引文件
private void writeCreateSubscriberMethods(BufferedWriter writer, List<ExecutableElement> methods,
String callPrefix, String myPackage) throws IOException {
for (ExecutableElement method : methods) {
//遍历事件方法列表
//获取方法参数列表
List<? extends VariableElement> parameters = method.getParameters();
//获取第一个参数类型对象
TypeMirror paramType = getParamTypeMirror(parameters.get(0), null);
//参数类型元对象
TypeElement paramElement = (TypeElement) processingEnv.getTypeUtils().asElement(paramType);
//获取方法名称
String methodName = method.getSimpleName().toString();
//获取参数类型类型,即事件类型
String eventClass = getClassString(paramElement, myPackage) + ".class";
//获取方法Subscribe注解对象
Subscribe subscribe = method.getAnnotation(Subscribe.class);
List<String> parts = new ArrayList<>();
//创建SubscriberMethodInfo
parts.add(callPrefix + "(\"" + methodName + "\",");
String lineEnd = "),";
if (subscribe.priority() == 0 && !subscribe.sticky()) {
//如果注解信息中优先级为0并且非粘性事件,则
if (subscribe.threadMode() == ThreadMode.POSTING) {
//如果线程模式为POSTING,则:
//创建:new SubscriberMethodInfo(methodName, eventClass)
parts.add(eventClass + lineEnd);
} else {
//线程模式不是POSTING,则:
//创建:new SubscriberMethodInfo(methodName, eventClass, ThreadMode.xxx)
parts.add(eventClass + ",");
parts.add("ThreadMode." + subscribe.threadMode().name() + lineEnd);
}
} else {
//如果注解信息中优先级不是0或者是粘性事件,则
//创建:new SubscriberMethodInfo(methodName, eventClass, ThreadMode.xxx, priority, sticky)
parts.add(eventClass + ",");
parts.add("ThreadMode." + subscribe.threadMode().name() + ",");
parts.add(subscribe.priority() + ",");
parts.add(subscribe.sticky() + lineEnd);
}
writeLine(writer, 3, parts.toArray(new String[parts.size()]));
if (verbose) {
processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE, "Indexed @Subscribe at " +
method.getEnclosingElement().getSimpleName() + "." + methodName +
"(" + paramElement.getSimpleName() + ")");
}
}
}

编译器注解处理器其实做的是跟运行时反射获取订阅者class的订阅事件方法列表有点类似,都是从订阅者class收集subscribe注解修饰的订阅事件方法列表。

编译器注解处理器主要做:

  • 收集本模块中被subscribe注解修饰的方法及其所属的类,并生成订阅者/事件方法列表map。
  • 一个模块一个索引类,生成本模块的索引类。
  • 将订阅者/事件方法列表map写入索引类中的订阅者/订阅信息map中,供运行时根据订阅者class快速获取订阅信息SubscriberInfo。

(4)从编译期生成的订阅者/事件关系文件中查找订阅方法列表

从编译期生成的订阅者/事件索引类文件中查找订阅方法列表的入口是findUsingInfo:

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
//从查找状态对象复用池中获取/创建一个新的查找状态对象
FindState findState = prepareFindState();
//初始化查找状态对象,设置订阅者class
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//从索引类中查找订阅者对应的定义信息对象
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
//如果订阅者订阅的订阅信息对象不为空,则:
//从订阅信息对象中获取订阅事件方法数组
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
//将数组转为list
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
//如果索引类中没有匹配的,则通过运行时注解反射获取订阅事件方法列表
findUsingReflectionInSingleClass(findState);
}
//递归从父类/父接口继续查找
findState.moveToSuperclass();
}
//返回订阅事件方法列表
return getMethodsAndRelease(findState);
}

findUsingInfo方法先从索引类中找到订阅者class对应的订阅事件方法列表,如果没有则转为运行时注解通过反射查找

从索引类中找到订阅者class对应的订阅信息对象的源码如下:

private SubscriberInfo getSubscriberInfo(FindState findState) {
if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
//订阅信息对象不为空,且有父级订阅信息对象,则:
SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
if (findState.clazz == superclassInfo.getSubscriberClass()) {
//如果订阅者class与父级订阅信息对象的class一样,则返回父级订阅信息对象
return superclassInfo;
}
}
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index : subscriberInfoIndexes) {
//遍历所有的索引类,即遍历所有模块的索引类(一个模块一个索引类)
//从索引类订阅者/订阅信息map中查找订阅信息对象
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
//找到则返回
return info;
}
}
}
return null;
}

  • 从编译期生成的订阅者/事件关系文件中查找订阅方法列表性能上比运行时注解反射获取事件方法列表更好。
  • 因为少了反射查找,编译器每个模块都会生成一个索引文件,该索引文件记录的是订阅者/订阅信息map。
  • 查找事件方法列表时时间在索引文件进行查找并转换为事件方法列表。

2、EventBus发布消息

EventBus基于观察者模式,所以其发布消息的流程大致为从观察者列表中找到匹配的观察者,并将消息传递给匹配的观察者处理。

EventBus是不是这样的处理逻辑呢,我们来看看其发布源码,EventBus的发布消息入口是post方法:

public void post(Object event) {
//从ThreadLocal获取当前线程的PostingThreadState对象
PostingThreadState postingState = currentPostingThreadState.get();
//发布消息队列
List<Object> eventQueue = postingState.eventQueue;
//将消息加入队列中
eventQueue.add(event);
if (!postingState.isPosting) {
//如果发布队列没有运行,则:
//标记发布状态所处的线程是否是主线程
postingState.isMainThread = isMainThread();
//标记正在发布消息
postingState.isPosting = true;
if (postingState.canceled) {
//已取消发布,则抛出异常
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
while (!eventQueue.isEmpty()) {
//遍历消息队列
//每次从队列删除首个消息,并进行发布
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}

post主要做的是将消息加入队列中,并遍历队列对每个消息进行一一发布,我们继续看下postSingleEvent:

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) {
//如果支持发布的消息的父级类型消息,则:
//从当前消息class找出其所有的父类/父接口类型列表,含本身
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++) {
//遍历消息类型列表
Class<?> clazz = eventTypes.get(h);
//对每一种消息类型进行发布消息
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
//如果不支持发布的消息的父级类型消息,则:
//直接发布当前消息
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
//......
}

postSingleEvent中分为两种情况,一种支持遍历当前消息的父级类型及本身作为消息,然后按照类型一一发布消息。

一种仅支持消息本身类型,则直接发布当前消息,我们继续往下看postSingleEventForEventType:

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//根据消息类型class从消息/订阅者列表map中找到订阅此消息的订阅者列表
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
//遍历此消息的订阅者列表
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
//调用postToSubscription将消息传递给订阅者进行处理
postToSubscription(subscription, event, postingState.isMainThread);
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}

postSingleEventForEventType中根据消息类型class从消息/订阅者列表map中找到订阅者列表,然后遍历订阅者列表,将消息传递给每个订阅者进行消息的处理。

我们继续往下看postToSubscription:

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case POSTING:
//如果订阅方法要求线程模式是POSTING,即在当前线程进行处理,则:
//直接调用invokeSubscriber方法回调订阅者的订阅方法处理消息
invokeSubscriber(subscription, event);
break;
case MAIN:
//如果订阅方法要求线程模式是MAIN,即要求在主线程进行处理,则:
if (isMainThread) {
//当前已是主线程,则直接调用invokeSubscriber方法回调订阅者的订阅方法处理消息
invokeSubscriber(subscription, event);
} else {
//当前不是主线程,则将使用handler转入主线程中处理
mainThreadPoster.enqueue(subscription, event);
}
break;
case MAIN_ORDERED:
//如果订阅方法要求线程模式是MAIN_ORDERED,即要求在主线程进行串行处理,则:
if (mainThreadPoster != null) {
//如果设置了主线程handler,则使用handler转入主线程中处理
mainThreadPoster.enqueue(subscription, event);
} else {
// temporary: technically not correct as poster not decoupled from subscriber
//如果没有设置主线程handler,则直接在当前线程处理
invokeSubscriber(subscription, event);
}
break;
case BACKGROUND:
//如果订阅方法要求线程模式是BACKGROUND,即要求在子线程进行处理,则:
if (isMainThread) {
//当前是主线程,则将消息转入子线程进行处理
backgroundPoster.enqueue(subscription, event);
} else {
//当前已子线程,则直接在当前线程处理
invokeSubscriber(subscription, event);
}
break;
case ASYNC:
//如果订阅方法要求线程模式是ASYNC,即要求在异步子线程进行处理,则:
//将消息转入子线程进行处理
asyncPoster.enqueue(subscription, event);
break;
default:
//无效的线程模型,抛出异常
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}

postToSubscription中主要是对线程模型进行处理:

  • POSTING:直接在当前线程处理。
  • MAIN:如果当前已是主线程,则直接处理消息;如果当前不是主线程,则通过handler转入主线程处理。
  • MAIN_ORDERED:主线程中串行处理,如果已经设置主线程Handler,则将消息加入消息队列等待loop进行调度;如果没有设置主线程Handler则直接在当前线程处理。
  • BACKGROUND:在子线程中处理,如果当前是主线程,则将消息转入子线程中再进行处理;如果当前已是子线程,则直接在当前线程处理。
  • *ASYNC:异步处理消息,不管当前是主线程还是子线程,都需要加入新的子线程中再进行处理。

直接处理消息的是通过invokeSubscriber方法,看名称就是表示反射调用订阅者的订阅方法处理消息,那是不是呢?我们看看其源码实现:

void invokeSubscriber(Subscription subscription, Object event) {
try {
//通过反射调用订阅者的订阅方法处理消息
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}

就是通过反射调用订阅者的订阅方法,将消息传递给订阅方法处理的;

(1)切换到主线程(MAIN)/主线程串行(MAIN_ORDERED)处理消息:

不管MAIN还是MAIN_ORDERED都是通过mainThreadPoster,将消息放入mainThreadPoster的队列中,mainThreadPoster其实就是主线程中的Handler:

//使用mainThreadSupport创建主线程消息发布器
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
public interface MainThreadSupport {
boolean isMainThread();
Poster createPoster(EventBus eventBus);
class AndroidHandlerMainThreadSupport implements MainThreadSupport {
private final Looper looper;
public AndroidHandlerMainThreadSupport(Looper looper) {
this.looper = looper;
}
@Override
public boolean isMainThread() {
return looper == Looper.myLooper();
}
@Override
public Poster createPoster(EventBus eventBus) {
//创建HandlerPoster作为mainThreadPoster
return new HandlerPoster(eventBus, looper, 10);
}
}
}

mainThreadPoster默认是通过MainThreadSupport.AndroidHandlerMainThreadSupport.createPoster创建的,也就是mainThreadPoster默认是HandlerPoster的实例对象,我们看看HandlerPoster:

public class HandlerPoster extends Handler implements Poster {
private final PendingPostQueue queue;
private final int maxMillisInsideHandleMessage;
private final EventBus eventBus;
private boolean handlerActive;
protected HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
super(looper);
this.eventBus = eventBus;
this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
queue = new PendingPostQueue();
}
public void enqueue(Subscription subscription, Object event) {
//将消息进行包装
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this) {
//将消息加入等待队列中
queue.enqueue(pendingPost);
if (!handlerActive) {
handlerActive = true;
//发送一个空消息
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
}
}
}
@Override
public void handleMessage(Message msg) {
boolean rescheduled = false;
try {
long started = SystemClock.uptimeMillis();
while (true) {
//死循环
//从等待队列中取出消息
PendingPost pendingPost = queue.poll();
if (pendingPost == null) {
synchronized (this) {
// Check again, this time in synchronized
pendingPost = queue.poll();
if (pendingPost == null) {
handlerActive = false;
//没有等待处理的消息,则退出循环
return;
}
}
}
//回调EventBus的invokeSubscriber处理消息
eventBus.invokeSubscriber(pendingPost);
long timeInMethod = SystemClock.uptimeMillis() - started;
if (timeInMethod >= maxMillisInsideHandleMessage) {
//没超过10毫秒重新发送空消息重新调度
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
rescheduled = true;
return;
}
}
} finally {
handlerActive = rescheduled;
}
}
}

  • 切换到主线程主要使用的是主线程的Handler实现。
  • 每次将消息加入等待队列,然后Handler发送一个空消息。
  • 在Handler的handleMessage中遍历等待队列消息出队并进行处理。

(2)切换到子线程(BACKGROUND)处理消息:

切换到子线程处理消息是通过backgroundPoster的enqueue方法,backgroundPoster是BackgroundPoster的实现类,我们看看其内部实现:

final class BackgroundPoster implements Runnable, Poster {
private final PendingPostQueue queue;
private final EventBus eventBus;
private volatile boolean executorRunning;
BackgroundPoster(EventBus eventBus) {
this.eventBus = eventBus;
queue = new PendingPostQueue();
}
public void enqueue(Subscription subscription, Object event) {
//将消息进行包装
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this) {
//将消息加入等待队列中
queue.enqueue(pendingPost);
if (!executorRunning) {
executorRunning = true;
//线程加入使用线程池中,等待回调run方法
eventBus.getExecutorService().execute(this);
}
}
}
@Override
public void run() {
try {
try {
while (true) {
//死循环
//从等待队列中获取等待处理的消息,1000表示如果队列为空等待1000毫秒
PendingPost pendingPost = queue.poll(1000);
if (pendingPost == null) {
synchronized (this) {
// Check again, this time in synchronized
pendingPost = queue.poll();
if (pendingPost == null) {
executorRunning = false;
//如果消息为空,则退出循环
return;
}
}
}
//调用EventBus反射调用订阅者的订阅方法处理消息
eventBus.invokeSubscriber(pendingPost);
}
} catch (InterruptedException e) {
eventBus.getLogger().log(Level.WARNING, Thread.currentThread().getName() + " was interruppted", e);
}
} finally {
executorRunning = false;
}
}
}

  • 切换到子线程处理消息主要是将消息加入等待队列,backgroundPoster本身是一个线程实现,将backgroundPoster本身加入线程池中运行。
  • backgroundPoster线程执行时遍历等待队列,从队列中取出等待处理的消息,通过eventBus.invokeSubscriber处理消息。

其中默认使用的线程池是缓存线程池:

executorService = builder.executorService;
public class EventBusBuilder {
private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();
ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}

(3)异步消息(ASYNC):

异步消息是通过asyncPoster的enqueue方法实现,asyncPoster是AsyncPoster的实例对象,它本身就是一个线程,我们看看其实现:

class AsyncPoster implements Runnable, Poster {
private final PendingPostQueue queue;
private final EventBus eventBus;
AsyncPoster(EventBus eventBus) {
this.eventBus = eventBus;
queue = new PendingPostQueue();
}
public void enqueue(Subscription subscription, Object event) {
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
//将消息加入队列中
queue.enqueue(pendingPost);
//线程加入线程池中
eventBus.getExecutorService().execute(this);
}
@Override
public void run() {
//每次取出一个消息
PendingPost pendingPost = queue.poll();
if(pendingPost == null) {
throw new IllegalStateException("No pending post available");
}
//调用EventBus反射调用订阅者的订阅方法处理消息
eventBus.invokeSubscriber(pendingPost);
}
}

异步消息的实现主要是将消息加入等待队列,然后将线程加入队列,线程执行是每次去等待队列的一个消息(队尾,先进先出原则),最后调用EventBus反射调用订阅者的订阅方法处理消息。

异步消息使用的线程池跟BACKGROUND模式使用的线程池是同一个,都是缓存线程池,与BACKGROUND的区别是:

  • 异步消息每次都将线程加入线程池中。
  • 而BACKGROUND则不需要每次。
  • 异步消息线程执行每次只去队尾消息进行处理,而BACKGROUND线程执行时会处理队列中的所有消息。

总结:

  • EventBus基于观察者模式,其首先需要先将订阅者与消息类型进行注册到一个消息/订阅者列表map中,发布消息时从map找找到对应的订阅者并调用相应的订阅方法进行处理消息。
  • 注册时通过订阅者class通过反射收集subscribe注解修饰的方法列表及注解信息,最终注册到消息/订阅者列表map中,这个运行时注解是先对耗时的;所以EventBus提供了编译期注解处理器来做个收集的工作,降低性能损耗。
  • 发布消息时根据消息类型class从消息/订阅者列表map中找到对应的订阅者列表并进行一一处理,同时根据注解中的线程模式进行线程切换处理。

图片


责任编辑:姜华 来源: Android开发编程
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